The present invention relates to a vehicle steer angle control system for controlling either or both of the front wheel steer angle and the rear wheel steer angle.
Conventional examples of such a steer angle control system are disclosed in U.S. Pat. No. 4,705,131 (corresponding to Japanese Patent Provisional Publication 60-161265), and U.S. Pat. No. 4,666,013 (corresponding to Japanese Patent Provisional Publication 60-161266). The conventional steer angle control systems of these examples are designed to determine front and rear wheel steer angles .delta..sub.f and .delta..sub.r by using the following equations: EQU .delta..sub.f (s)=(af.sub.0 +af.sub.1 s).theta.(s) (1) EQU .delta..sub.r (s)=(ar.sub.0 +ar.sub.1 s).theta.(s) (2)
where .delta..sub.f (s) and .delta.r(s) are the Laplace transforms of the front and wheel steer angles .delta..sub.f and .delta.r, .theta.(s) is the Laplace transform of the steering wheel angle .theta., af.sub.0, af.sub.1, ar.sub.0 and ar.sub.1 are coefficients whose values are determined by a vehicle speed, and is a Laplace transform variable.
When lags are involved in actuating systems for steering the front and rear wheels, then it is advisable to use the following equations including a term of a second derivative; EQU .delta..sub.f (s)=(af.sub.0 +af.sub.1 s+af.sub.2 s.sup.2).theta.(s)(3) EQU .delta..sub.r (s)=(ar.sub.0 +ar.sub.1 s+ar.sub.2 s.sup.2).theta.(s)(4)
where af.sub.2 and ar.sub.2 are coefficients whose values are determined by the vehicle speed.
Therefore, these steer angle control systems need differentiating means for producing an output derivative signal proportional to the derivative of an input signal, such as the steering wheel angle .theta., with respect to time. However, the differentiating process employed in the conventional steer angle control systems is not accurate enough for the following reason.
Especially when a digital sensor such as a rotary encoder is employed for sensing the steering wheel angle .theta., the derivative signal obtained from the sensed values of the steering wheel angle .theta. tends to have periodic sharp changes like pulses whereas the steering wheel angle .theta. is changing at a constant speed. Accordingly the derivative must be constant. FIG. 4 shows one example. In this example, the value of the steering wheel angle .theta. is read in at time intervals .DELTA.t equal to 5 msec, and the resolution .DELTA..theta. of the steering angle sensor is 1 deg. In this case, the resolution of the first derivative .theta. is given by 1 deg/5 msec=200 deg/sec, and the resolution of the second derivative .theta. is 1 deg/(5 msec).sup.2 =40000 deg/sec.sup.2. As a result, pulses are formed in the first derivative signal and the second derivative signal as shown in FIG. 4. In FIG. 4, the true value of the steering wheel angle .theta. is indicated by an inclined straight line alpha while measured values of the steering wheel angle .theta. are indicated by small circles distributed along the line alpha. Lines beta and gamma indicate true values of the first derivative .theta. and the second derivative .theta.. Small circles along the line beta and the line gamma indicate, respectively, first derivative values and second derivative values obtained from the measured values of the steering wheel angle .theta.. As shown in FIG. 4, pulses are formed in the first derivative signal .theta. consisting of a sequence of the first derivative values, and the second derivative signal .theta. consisting of a sequence of the second derivative values.
Consequently, the conventional steer angle control systems tend to be inaccurate in determining the front and rear wheel steer angles by solving Equations (1) and (2) or Equations (3) and (4), so that it is difficult to achieve the desired steering steer angle control.
Systems arranged to filter the measured values of the steering wheel angle .theta. are also inaccurate because delays due to a filter are introduced in the proportional terms af.sub.0 .theta. and ar.sub.0 .theta. which are immune from pulse-like fluctuations.